The present invention is related generally to vehicle wheel alignment systems, and in particular, to a method for compensating for axial misalignment between wheel-mounted alignment sensors and the wheel axis of rotation by rolling the wheel-mounted sensors together with the associated vehicle wheels about a limited range of movement.
In any wheel alignment system the orientation of the axis of rotation of the vehicle wheels must be determined before alignment angles of the wheels can be calculated. In an ideal world, the alignment system sensors would be mounted to the wheels such that they would be perfectly lined up with the axis of rotation of the wheels and measurements could be directly obtained. However, it is necessary to accommodate the reality of an imperfection in mounting a sensor on a wheel. This is accomplished by performing a compensation procedure which consists of observing camber angle and/or toe angle changes measured by the sensor at different rotational positions of the associated wheel, caused by any eccentricity in the mounting of the sensor to the wheel.
There are many different types of wheel alignment systems currently available. For example, as is shown in FIG. 1, a conventional 6-sensor system utilizes sensors mounted at each vehicle wheel, with an extra set of cross-looking sensors associated with the front vehicle wheels. FIG. 2 illustrates a variation of the sensor system of FIG. 1, in which illuminated lights are disposed on one vehicle wheel (typically a rear wheel), and their relative positions are observed by sensors mounted on an adjacent wheel (typically a front wheel) to determine wheel alignment angles.
Before electronic tilt sensors were commonly available, compensation for the eccentric mounting of a sensor assembly onto a vehicle wheel was accomplished mechanically. The time consuming mechanical compensation process requires a series of incremental adjustments to adjusting screws on the sensor mounting after the fitment to a wheel assembly, such that a bubble level on the mounting is maintained a constant position as the wheel assembly is rotated. The process requires the wheel to be jacked up off a supporting surface, and requires several full 360 degree rotations of the wheel to ensure accurate compensation is achieved.
Some conventional wheel alignment systems employing wheel-mounted sensors use a compensation procedure that requires readings at only two wheel rotation positions that are 180 degrees apart. The offset for the axis of rotation is found by averaging the readings from each position. While this method is simple, error is introduced if, during the remainder of the alignment procedure, the wheels are allowed to roll away from the rotational position at which the compensation setting was calculated. Also, the vehicle must be jacked up above the supporting surface to achieve the required 180 degrees of wheel rotation.
Another known compensation method for wheel-mounted sensors requires measurements at three separate wheel rotational positions. The readings from the sensors are then fitted to a sine wave, which is analyzed to determine the amplitude and phase of the mounted sensor error with respect to the axis of rotation of the wheel. Once the relationship is established, the wheel can be subsequently rolled to any rotational position and an accurate compensation value determined from the corresponding values of the fitted sine wave. However, this method requires the wheel to be jacked up off the supporting surface to accomplish at least 240 degrees of wheel roll, as described in U.S. Pat. No. 5,052,111 to Carter et al.
An alternate method of sensor compensation requires a very accurately machined wheel adapter to be used in conjunction with special machined provisions on the wheel and brake hub. When the adapter is mounted on the wheel in a precise and predetermined mounting using the machined provisions, the axis of the adapter is in line with the axis of rotation of the wheel. A pre-compensated sensor is then mounted in the perfectly aligned socket of the wheel adapter and readings can be obtained without the need to jack up the car and roll the wheel. However, any foreign objects in, or damage to, the interface between the wheel adapter and the machined provisions on the hub will cause erroneous readings. This method is expensive for both the car manufacturer and the alignment equipment manufacturer due to the precision machining required.
The above compensation methods are generally useful, but could be time consuming or prone to error. A more efficient rolling compensation method was introduced along with machine-vision style wheel alignment systems employing optical targets mounted to each wheel of a vehicle, such as those shown in U.S. Pat. Nos. 5,535,522, 5,724,743, and 5,943,783. Generally, machine-vision vehicle wheel alignment systems employ cameras or imaging sensors to view the front wheels and rear wheels of a vehicle disposed on a vehicle lift rack or other supporting surface. Optical targets are affixed to the respective vehicle wheels, and are observed by the cameras or imaging sensors, which are in turn coupled to a computer having an associated memory and display. The computer processes the resulting images, is programmed to process mathematical transforms on the resulting data from which wheel alignment angles are determined.
The rolling compensation procedure, illustrated generally by the rearward and forward rolling movement of the vehicle wheel on the supporting surface as shown in FIGS. 3A-3C, can be accomplished with a minimal wheel rotational movement of only 30 to 60 degrees. All four wheels are rolled at the same time by pushing (or pulling) the vehicle and no jacking is required. The procedure is quick to perform and accurately compensates for errors in the mounting of a wheel adapter. However, this rolling compensation method has traditionally been restricted to systems using machine-vision technology to observe the motion of the wheel-mounted optical targets during the rolling movement.
Wheel alignment system which utilize machine-vision technology require at least one camera with a two-dimensional imager array and at least one wheel-mounted target. The typical system has the cameras mounted remotely from the vehicle while the targets are affixed to the wheels of the vehicle. In these systems, the method of compensation disclosed in the above patents acquires images of the target in multiple positions during the wheel rotation procedure. The images are processed by the computer to identify a surface of revolution of each target generated by the rotational movement of the associated wheel. An axis of rotation for the surface of revolution is identified, and established as the rotational axis of the associated wheel. Alternatively, the conical path traced by the surface normal of the “claw” plane can also be used to find the axis of rotation, as is described in U.S. Pat. No. 5,969,246 to Jackson.
Recently, a hybrid wheel alignment system has been introduced where cameras are mounted with adapters onto the rear wheels of a vehicle, and are configured to observe optical targets which are mounted with adapters onto the front wheels of the vehicle, as shown in U.S. Pat. No. 7,313,869 B1 to Rogers. Compensation with this system is accomplished by using separate procedures for the front sensors and for the rear sensors, effectively doubling the operators work load. The rear sensors, containing the cameras, also incorporate camber transducers which are used to perform a conventional compensation requiring anywhere from 180 to 360 degrees of wheel roll. The front wheels are then compensated with the standard machine-vision methods using the surface of revolution generated by the offset targets as the wheels roll. These complicated compensation requirements of the hybrid system are a major disadvantage. If the vehicle is rolled for compensation then it must be rolled far enough to complete at least 180 degrees of wheel rotation as required to compensate the rear sensors. If it is not rolled, then jacking the vehicle above the supporting surface is necessary while the front and rear wheels are rotated separately for the compensation procedures. These limitations exist because there is no means available at the rear wheel to generate the surface of revolution necessary for machine vision compensation unless an additional fixed reference target is present in the field of view. Hence, the hybrid system as implemented by the '869 Rogers patent provides no operational compensation advantage over a conventional wheel-mounted alignment systems due to the restricted compensation methods required for the different types of sensors.
However, complete machine-vision alignment systems utilizing only the rolling compensation procedures do provide an operational advantage over traditional wheel-mounted systems in that the time required for sensor compensation is dramatically shorter. This is achieved by the use of the rolling compensation methods which compensate all four wheels simultaneously without the need to jack up the vehicle. In addition, the amount of wheel rotation required for compensation of a machine-vision alignment system is 60 degrees or less, as compared to the wheel-mounted sensor systems which require a minimum of 180 degrees.
Accordingly, it would be advantageous to provide a method for compensating axial misalignment between wheel-mounted alignment sensors and the axis of rotation for an associated wheel which can compensate sensors on all wheels of a vehicle simultaneously without requiring jacking of the vehicle off a supporting surface, and which only requires a wheel rotation through an arc of 60 degrees or less.